This is now possible since the code allocating OutputFile can see
sizeof(Muxer). Avoids the overhead and extra complexity of allocating
two objects instead of one.
Similar to what is done e.g. for AVStream/FFStream in lavf.
ffmpeg_opt.c currently contains code for
- parsing the options provided on the command line
- opening and initializing input files based on these options
- opening and initializing output files based on these options
The code dealing with each of these is for the most part disjoint, so it
makes sense to move them to separate files. Beyond reducing the quite
considerable size of ffmpeg_opt.c, this will also allow exposing muxer
internals (currently private to ffmpeg_mux.c) to the initialization
code, thus removing the awkward separation currently in place.
